1. Technical Field
The present invention generally relates to a semiconductor memory device, and more particularly, to a variable resistance memory device and an operating method thereof.
2. Related Art
In general, a semiconductor memory device is classified into a volatile memory device and a nonvolatile memory device. The volatile memory device loses data stored therein when power supply is cut off, but the nonvolatile memory device maintains data stored therein even though power supply is cut off. The nonvolatile memory device may include various types of memory cells.
The nonvolatile memory device may be divided into a flash memory device, a ferroelectric RAM (FRAM) using a ferroelectric capacitor, a magnetic RAM (MRAM) using a tunneling magneto-resistive (TMR) layer, a phase change memory device using chalcogenide alloys and the like, depending on the structure of memory cells. In particular, the phase change memory device is a nonvolatile memory device using phase change based on temperature change, that is, resistance change. For this reason, the phase change memory device is also referred to as a variable resistance memory device.
The phase change memory device includes memory cells formed of a phase change material, for example, a chalcogenide alloy which is a germanium(Ge)-antimony(Sb)-tellurium(Te) mixture (GST) (hereafter, referred to as a GST material). The GST material has an amorphous state having a relatively high resistivity and a crystalline state having a relatively low resistivity. The memory cell of the phase change memory device may store data ‘0’ corresponding to the amorphous state and data ‘1’ corresponding to the crystalline state. The GST material may be heated to program the data corresponding to the amorphous state or the crystalline state into the memory cell of the phase change memory device. For example, the amorphous state or the crystalline state of the GST material may be controlled by adjusting the magnitude of a current for heating the GST material and the application time of the current.
However, as the time elapses after the program operation, resistance drift may occur in the GST material forming the memory cell of the phase change memory device. When the resistance drift occurs, the resistance value of the GST material does not maintain a fixed value, but increases. The resistance change of the GST material may reduce a sensing margin of the memory cell, thereby reducing the reliability of the phase change memory device.